Introduction
Respiratory disease has become very common among people throughout the world and accounts for about 414.6 million cases reported around the world. Severe lung problems are one of the major causes of mortality. The British lung foundation states that one person dies every five minutes in the UK due to severe lung disease, the third most common cause of death in the UK. The increased incidence of lung disease and the rate of mortality caused by them has created a rising demand for accurate diagnosis and patient monitoring. The development of ultrasound and other advanced imaging techniques has improved the examination and treatment performance to a large extent among clinicians. The ultrasound imaging techniques have high specificity ranging from 95 to 100 % and sensitivity ranging from 81 to 97 %. In this article, we will review the recent advances in ultrasound technology and its ability to identify various disease processes affecting the lungs.
What Is the Working Principle of Ultrasound Imaging?
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Ultrasound machines have a handheld transducer that can emit ultrasound waves and detect the waves echo after hitting a body surface. The transducer consists of an active element of special ceramic crystal materials called piezoelectrics. Applying an electric field onto the piezoelectric will produce ultrasound waves; similarly, they will produce an electric field when a sound wave is applied.
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In an ultrasound scanner, the transducer produces a beam of ultrasound wave to the body, which then gets reflected by hitting tissue boundaries, surfaces, and fluids and is received by the transducer. When they hit the transducer, the echoed waves produce electrical signals carried to the ultrasound scanning machine. By determining the speed of sound and the time taken by the waves to return to the transducer, the distance from the transducer to the tissue is calculated. The distance obtained by calculations is then used to generate a two-dimensional image of the organs and tissues.
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During an ultrasound examination, a gel is applied to the skin to avoid the formation of air pockets that will block the passage of ultrasound waves into the body.
What Are the Uses of Lung Ultrasound?
Ultrasound imaging has become a very common practice in the field of biomedical imaging. They act as an additional diagnostic tool and have unique advantages, they are non-ionizing and are often easily portable. Diagnostic ultrasounds are used for imaging organs within the body. However, it is not perfect for imaging organs that contain air, like the lungs and some bones. In certain conditions, when the lung is partially or filled with fluid, ultrasound is a perfect diagnostic tool for imaging the fluid-filled lungs. The image is displayed in 2D, 3D, or 4D (3D in motion). Several advances in ultrasound imaging have expanded the use of lung ultrasound to identify deep lesions apart from the detection of pleural effusion.
When the lungs get filled with fluid due to a pathological condition, the air content decreases, and the lung density increases due to transudate in the lungs; this phenomenon will create artifacts that will help the clinician to identify the peculiar condition of the lungs and determine the fluid overload to the body.
The ratio of air-liquid inside the lungs changes in pathological conditions and creates strange artifacts in ultrasound images. The artifacts are presented in two forms;
- A- lines: They are horizontal linear structures caused by the reflection of sound waves multiple times due to the difference in acoustic impedance (ratio of acoustic pressure to the particle speed) between the lung and the pleura; they indicate a healthy lung.
- B- lines: They appear as a vertical line from the pleural line resembling a comet tail. The artifact arises from decreased lung aeration and is indicative of several diseases. The position of the B-line from the pleural line also helps determine whether the line detected is merely any other organ or the B-line.
These lines tell much about the pathological condition, so diagnosing begins with identifying and quantifying the linear artifacts. In some studies, the pleural line is used as a reference for the identification of the artifact lines. Some pathological conditions of the lungs that are identified with lung ultrasounds are as follows.
- Pneumothorax: The lung ultrasound displays superpositioned parallel lines that do not have motion characteristics.
- Pneumonia and COVID-19: There is a presence of abnormal pleural lines, B-lines fusion, and pleural effusion. Vertical artifacts are seen in the early stages of COVID-19 pneumonia.
- Chronic Obstructive Pulmonary Disease (COPD): Presence of pulmonary A-line and absence of right ventricular overload.
- Acute Respiratory Distress Syndrome (ARDS): The disappearance of A-line, abnormal pleural line, and lung consolidation.
- Lung Cancer: Captures the peripheral and deep lesions and shows their boundaries and anatomical relationship with the surrounding structures.
- Congenital Diaphragmatic Hernia (Cdh): Partial disappearance of the pleural line on the affected side.
What Are the Recent Advances in the Lung Ultrasound?
The lung ultrasound examination has seen several advances in the last few decades. Currently, their application reaches far beyond the investigation of pleural effusion alone. Several developments have been introduced to improve image acquisition and processing capabilities; the portability was also increased with the advent of new smaller machines with high-quality imaging.
The advances in lung ultrasound technology have been mainly in computational imaging and image analysis methods. The advances are of two types:
1) Model-Based Method - They provide a framework for labeling lung ultrasound images in cases where machine learning data is unavailable.
2) Data-Driven Method - They capture more complex data patterns and provide better image analysis performance.
The advancement in imaging techniques has not only impacted the diagnosis, treatment, prognosis, and patient management. It became an auxiliary diagnosis tool and visual stethoscope for radiologists. However, the pace with which studies for the advancement and development of lung ultrasound are moving is not satisfactory and has yet to reach maturity. Several types of research are being conducted to provide reference and inspiration for developing more efficient and accurate lung ultrasound imaging approaches.
Conclusion
In conclusion, lung ultrasound has tremendous potential for use in clinical practice. It has shown promise for diagnosing and monitoring respiratory diseases. Its use in the ICU (intensive care unit) setting has been particularly beneficial due to its ease of use and quick results. Further research is needed to define the indications for their use better and optimize the technique for improved accuracy and reliability.
